1. Technical Field
The present invention relates to an optical-fiber-characteristic measuring apparatus and an optical-fiber-characteristic measuring method, and more particularly, an optical-fiber-characteristic measuring apparatus and an optical-fiber-characteristic measuring method which utilize a stimulated Brillouin scattering phenomenon that occurs in a measurement-target optical fiber to sense the distribution of strains or the like applied to the optical fiber.
2. Description of Related Art
Brillouin scattering which occurs in an optical fiber changes in accordance with a strain applied to the optical fiber. A technology which measures the distribution of strains along the lengthwise direction of an optical fiber using such a phenomenon has been developed. That technology enables the measurement of the size of a strain by measuring the frequency change in Brillouin scattering light, and enables the pinpointing of the strained portion of the optical fiber by measuring the time until the Brillouin scattering light comes back. Therefore, by causing an optical fiber to run across a construction, such as a bridge, a bridge column, a building, or a dam, and the material of a wing, fuel tank or the like of an aircraft, the distribution of strains applied to such a construction and a material can be detected. Based on the distribution of strains, the deterioration and the aged deterioration of the construction and the material can be known, resulting in disaster prevention and accident prevention.
According to a method of measuring a strain distribution amount which has been known so far, light pulses are input into an optical fiber, and a Brillouin scattering light scattered backwardly is measured in a time-resolving manner. According to such a method of measuring time regions by light pulses, however, the measurement time is long (taking several minutes to ten minutes), and the spatial resolution is limited (maximum: 1 m), so that the method is insufficient for an application where various constructions are dynamically managed. Therefore, users have been seeking a break-through technology which has a high spatial resolution and which can specify a portion where a strain is applied at a further short time.
To respond to such a need, the inventors of the present invention propose, unlike the conventional time-resolving measurement method of light pulses, a technology of measuring the distribution of Brillouin scatterings along the lengthwise direction of an optical fiber by controlling the interference condition of continuous light in patent literature 1 and patent literature 2, and acquire a patent for that technology. The technology is known as BOCDA (Brillouin Optical Correlation Domain Analysis), and achieves a 1 cm spatial resolution and a sampling rate of approximately 60 Hz, thereby attracting attention.
An explanation will now be given of the principle of Brillouin scattering. In a case where light is input into a general optical fiber, in ultrasound generated by thermal vibration of the glass molecules of the optical fiber material, ultrasound having a wavelength half of the wavelength of an input light is generated. Periodical changes in the refraction index of glass originating from the ultrasound work as a Bragg diffraction grating, and reflect the light backwardly. This is a Brillouin scattering phenomenon. The reflected light is subjected to a Doppler shift depending on the speed of the ultrasound, and because the size of the frequency shift changes in accordance with an expanding and contracting strain applied to the optical fiber, the strain can be detected by measuring the shift amount.
Specifically, as shown in the principle diagram of FIG. 22, two propagation waves having different frequencies, i.e., a stronger pump light and a weaker probe light, are oppositely propagated into a measurement-target optical fiber FUT from a light source 101 including a semiconductor laser, a signal generator, or the like. At this time, as a particular phase (frequency) matching condition is satisfied between the pump light and the probe light (fpump=fprobe+VB, where fpump is the center frequency of the pump light, fprobe is the center frequency of the probe light, and VB is a Brillouin frequency), acoustic phonons which scatter photons from the pump light to the probe light are generated by the interaction between both waves. This causes the amplification of the probe light as stimulated Brillouin scattering. However, when the frequency difference between the pump light and the probe light varies largely, stimulation is suppressed.
As explained in, for example, patent literature 1, the basic principle of the BOCDA method is to periodically generate Brillouin scattering having an intensive and sharp correlation peak and depending on a position along the measurement-target optical fiber FUT by performing the same frequency modulation on the pump light and the probe light propagated oppositely. Thus, according to the BOCDA method, light output from the light source 101 serves as continuous oscillation light, the oscillation frequencies thereof are changed by the repeated waveform of a sine wave, and a non-illustrated optical frequency modulator changes the center frequency fprobe of the probe light in such a way that the difference between the center frequency fprobe of the probe light and the center frequency fpump of the pump light becomes close to the Brillouin frequency VB. Accordingly, at most positions where the phases of the pump light and the probe light become asynchronous and the correlation of both lights is low, scattered light is widespread and weakened across a frequency range, and on the other hand, at a specific narrow position (correlation position) of the order of a cm where the phases of the pump light and the probe light are synchronous and the correlation of both lights is high, stimulated Brillouin scattering having an original peak spectrum occurs. Shifting the correlation position enables a measurement of the distribution of strain by Brillouin scattering.
FIG. 22 shows the spectrum shape of stimulated Brillouin scattering occurred at each position in the measurement-target optical fiber FUT. Note that BG means a Brillouin gain, and ΔV means a frequency difference between the pump light and the probe light. Due to frequency-modulated light from the light source 101, a stimulated scattered spectrum is widespread over a frequency axis and is weakened at most positions of the measurement-target optical fiber FUT, but a relative frequency difference between the pump light and the probe light becomes constant at a specific position (correlation position), and stimulated Brillouin scattering having an original Lorentz spectrum occurs.
As explained above, in a case where sinusoidal frequency modulation is performed on the light source 101, a spatial resolution Δz and a measurement range (interval between adjoining correlation positions) dm of the BOCDA method can be given by the following equations 1 and 2, respectively.Δz=(Vg·ΔVB)/2πfm·Δf  [Equation 1]dm=Vg/2fm  [Equation 2]
Note that Vg is the speed of light in the measurement-target optical fiber FUT, ΔVB is the Brillouin gain line width of the measurement-target optical fiber FUT (up to 30 MHz for general fibers), fm is a frequency-modulation frequency of the light source 101, and Δf is amplitude of frequency modulation. As explained in patent literature 1, to cause only a single position where a correlation peak occurs (correlation position) to be present within a measurement range, equation 2 is used and the measurement range dm is adjusted. In this case, because the measurement range dm is inversely proportional to the frequency-modulation frequency fm, if the frequency-modulation frequency which is the speed of frequency modulation on the light source 101 is lowered to make the frequency-change gradual, the interval between adjoining 0-order to n-order correlation positions, and the measurement range dm can be extended. However, the 0-order correlation position where the optical path difference between the probe light and the pump light does not change by merely changing the frequency-modulation frequency fm. Accordingly, to change all of the 0-order to n-order correlation positions while maintaining the measurement range dm, an optical delay device may be inserted into the optical path of either of the probe light or the pump light. By changing the frequency-modulation frequency fm of the light source 101 in this manner, a correlation position where a Brillouin gain spectrum is measured can be changed.
In a case where the frequency-modulation frequency fm is lowered to extend the measurement range dm, in this case, as is apparent from equation 1, the spatial resolution Δz is deteriorated and becomes a large value. Therefore, to extend the measurement range dm while maintaining the spatial resolution Δz high, the amplitude (modulation amplitude) Δf of the light source 101 may be increased within a range where the spectra of the probe light and the pump light do not overlap with each other.
According to the BOCDA method, measuring means for detecting the Brillouin gain BG of the probe light at an end of the measurement-target optical fiber FUT and recording it while sweeping around the frequency difference between the pump light and the probe light and roughly corresponding to the Brillouin frequency VB is used. The spectrum shape (output of this measurement method) of a Brillouin gain obtained from an end of the measurement-target optical fiber FUT is a sum of Lorentz spectra (actual signal) generated at a correlation position and the integrated value (noise) of a wide spectrum generated at another position. This will be explained with reference to FIG. 23, and in FIG. 23, the spectrum shapes of individual detection outputs are shown for a case where no strain is applied to the correlation position (upper part) and for a case where a strain is applied to the correlation position (bottom part). A spectrum shape can be can be divided into the component of an actual signal S1 from a correlation peak, and the component of a noise S2 from each of all positions other than the correlation position. In a case where a strain, a temperature change, or the like is applied to the position of the correlation peak, as shown in the bottom part of FIG. 23, only the actual signal S1 shifts from the original frequency difference ΔV. That is, the peak of the Brillouin gain spectrum generated at the correlation position represents a strain at the correlation position as the actual signal S1. Accordingly, when the correlation position is swept while changing the frequency (fm) of the frequency modulation of the light source 101, and when the peak frequency of a spectrum at each correlation position is specified, it becomes possible to measure the distribution of strains along the measurement-target optical fiber FUT.
Patent Literature 1: Japanese Patent Publication No. 3667132
Patent Literature 2: Japanese Patent Publication No. 3607930
According to the foregoing device and method which measure the distribution of characteristics like strains of the measurement-target optical fiber FUT, the longer the measurement-target optical fiber FUT becomes, the wider the measurement range dm must be extended, resulting in an integration of unnecessary components weakened at respective positions other than the correlation position, thus increasing the noise S2. That is, because the level of the noise S2 detected by the measuring means is a sum of all Brillouin gain spectra from all non-correlation positions, the wider the measurement range dm becomes under a certain spatial resolution Δz, the more the peak-to-peak ratio (SNR) between the actual signal and the noise decreases. Accordingly, as particularly shown in the bottom part of FIG. 23, at a position where the shift frequency of the accrual signal S1 due to a strain is large, the signal peak thereof becomes smaller than the level of the noise S2, so that the measurement of the distribution of strains becomes difficult.
As explained above, the background component of the noise S2 deteriorates the precision of measuring the distribution of characteristics of the measurement-target optical fiber FUT, and limits the measurement range dm, so that there is a demand to suppress the noise S2 which is an unnecessary component.
The present invention has been made in view of the foregoing problems, and it is an object of the invention to provide new optical-fiber-characteristic measuring apparatus and optical-fiber-characteristic measuring method which effectively suppress the noise level of integrated unnecessary components from non-correlation positions to improve the measurement precision, and to extend a measurement range.